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ARROW-9775: [C++] Automatic S3 region selection

Once looked up, the region of a given bucket is cached for the lifetime of the process.
(the cache size is unbounded, which shouldn't be a problem in practice)

S3Options::FromUri() now resolves the region for the bucket if it wasn't given as a Uri query parameter.

Closes #8205 from pitrou/ARROW-9775-s3-auto-region

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>